The majority of the world's vehicle license plates are retro-reflective by design. This means they reflect a large proportion of incident light back in the direction of the incident beam. They are designed this way in order to make them readable at night by shining a light on them. A car following another car can read the front car's rear license plate even at night because the plate reflects the following car's headlight beams back. The taillights of the car in front are typically not bright enough to overpower and obscure the light that is retro-reflected from the license plate of the car in front. It is however difficult or impossible to make out the front license plates of approaching traffic at night, because the headlights of the approaching traffic does overwhelm the visible light reflected from the front license plates. Even in reading the rear license plate of following traffic, the reader should be within an angle of 5 degrees of the illuminating source in order for the retro-reflecting surface to operate effectively and yield a sufficiently informative amount of light and contrast. Non-retro-reflective surfaces obey Snell's law where the light is reflected at an angle to the normal of the surface opposite but equal to the angle of the incident light. The retro-reflective nature of these plates is mainly for high visibility.
In trying to image a vehicle's license plates with CCD cameras several problems can occur. Firstly, at night there may be no ambient light to be reflected from a license number plate so an illuminator must be used. Secondly, if the vehicle's headlights are on, the cameras can be blinded by the glare from the headlights overriding any light information reflected from the license plate itself. The CCD elements become saturated and the stored charge over spills into adjacent cells resulting in a picture which is smeared and in which the license plate's numbers and letters either cannot be seen at all or are illegible. The CCD will generally try to handle this amount of light by increasing the shutter speed with the result that it can image the headlights perfectly but the gain is so reduced that the number plate disappears from the image. If the vehicle is moving fast it may travel some distance in the time that the CCD is collecting light. This can result in a blurred image.
An imaging system of this nature has several other requirements. If it is to be mounted on a highway, a toll both, or an unmanned security gate, there needs to be a reasonable lifetime. The requirement to cordon off one or more lanes of traffic and to hire a cherry picker lift to replace a bulb is undesirable. For mounting on a bridge or overpass over a highway there is a certain requirement for the range from camera to vehicle, depending on the height of the bridge. Also the unit cannot be at too acute an angle to the license plate, or the alphanumeric symbols on the license plate, will be too skewed for optical character recognition software to decipher them. The system needs also to cope with the elements, extreme cold, heat and rain.
The invention works well with retro-reflective plates because most such plates not only reflect visible light as intended, but also reflect infrared illumination, with the alpha-numeric symbols used in most license plate territories giving good contrast in the infrared spectrum as well as in visible light. Another background factor that creates the need for this invention, yet allows it to work effectively is that the headlights of vehicles produce most of their focussed output in the visible light spectrum, whereas their infrared output is more limited and diffuse.